The flat portions imply that the temperature is not changing, and correspond to the processes of
melting and boiling.
Reactions that absorb heat energy are said to be endothermic, while those that release heat are said
to be exothermic. An adiabatic process is one in which no heat exchange occurs (no heat goes into
or out of the system). Melting and vaporization are therefore endothermic, while freezing and
condensation are exothermic processes. An isothermal process is one in which the temperature of
the system remains constant.
CALORIMETRY
Calorimetry is an experimental technique that measures the heat change associated with a process.
In constant-volume calorimetry, the volume of the container holding the reacting mixture does not
change during the course of the reaction. The heat of reaction is measured using a device called a
bomb calorimeter. This apparatus consists of a steel bomb into which the reactants are placed. The
bomb is immersed in an insulated container holding a known amount of water. The reactants are
electrically ignited and heat is absorbed or evolved as the reaction proceeds. The heat of the
reaction, qrxn, can be determined as follows. Since no heat enters or leaves the system, the net heat
change for the system is zero; therefore, the heat change for the reaction is compensated for by the
heat change for the water and the bomb, which is easy to measure.